Switching power regulators or converters (SPC) are often used in electronic systems to convert a direct current (DC) voltage into a different DC voltage. Or, they may be used to convert an alternative current (AC) voltage into a DC voltage, or even converting a DC voltage into an AC voltage. They are widely used in both portable and non-portable applications for a large variety of applications and of power and voltage ranges. There are numerous architectures for each application such as Buck (Step Down), Boost (Step Up), H-Bridge, and Fly Back. Many common products use SPC's, including digital cameras, cell phones, MP3 players, and so on, and in many instances there are several different SPC's in one such product, each with their own particular load and controllers and their particular sets of specifications.
One approach to implementing a step-down (or ‘Buck’) DC-DC converter is called the Pulse Width Modulation (PWM) regulation method. In a PWM regulator, the oscillation frequency is kept constant, while the duty cycle is varied to control output voltage. Another approach is Pulse Frequency Modulation (PFM), where, for example, the value of the on-time can be kept constant and regulation can be achieved by varying the value of the off-time (defined as Toff=T−Ton). Or, alternatively, off time (Toff) can be kept constant and Ton can be varied to control the output voltage. In both cases for a PFM regulator the period of clock (T) is changed while either Toff or Ton is kept constant.
One of the problems with either approach occurs when the system is initially powered on. Many designs include an inductor which can create issues due to the ‘short circuit’ behavior of an inductor at power on, Absent current limiting at power on, many designs of SPC's can supply a sufficiently high current to damage the inductor and potentially other components. Thus, it is important to control the value of Ion during the initial turn on.
One of the common schemes to control the initial value of Ton and to limit the initial current is to add various components to the design. In one approach, a capacitor is added, and by slowly charging the capacitor and by using the voltage on the capacitor to slowly increase the value of Ton, a safe power on is achieved.